On Sun, Jan 30, 2022 at 09:07:16AM -0800, Florian Fainelli wrote:

On 1/30/2022 5:59 AM, Ansuel Smith wrote:

quoted

quoted

Hi,
sorry for the delay in sending v8, it's ready but I'm far from home and
I still need to check some mdio improvement with pointer handling.

Anyway I have some concern aboutall the skb alloc.
I wonder if that part can be improved at the cost of some additional
space used.

The idea Is to use the cache stuff also for the eth skb (or duplicate
it?) And use something like build_skb and recycle the skb space
everytime...
This comes from the fact that packet size is ALWAYS the same and it
seems stupid to allocate and free it everytime. Considering we also
enforce a one way transaction (we send packet and we wait for response)
this makes the allocation process even more stupid.

So I wonder if we would have some perf improvement/less load by
declaring the mgmt eth space and build an skb that always use that
preallocate space and just modify data.

I would really love some feedback considering qca8k is also used in very
low spec ath79 device where we need to reduce the load in every way
possible. Also if anyone have more ideas on how to improve this to make
it less heavy cpu side, feel free to point it out even if it would
mean that my implemenation is complete sh*t.

(The use of caching the address would permit us to reduce the write to
this preallocated space even more or ideally to send the same skb)

I would say first things first: get this patch series included since it is
very close from being suitable for inclusion in net-next. Then you can
profile the I/O accesses over the management Ethernet frames and devise a
strategy to optimize them to make as little CPU cycles intensive as
possible.

Don't know if it's correct to continue this disccusion here.

build_skb() is not exactly a magic bullet that will solve all performance
problems, you still need the non-data portion of the skb to be allocated,
and also keep in mind that you need tail room at the end of the data buffer
in order for struct skb_shared_info to be written. This means that the
hardware is not allowed to write at the end of the data buffer, or you must
reduce the maximum RX length accordingly to prevent that. Your frames are
small enough here this is unlikely to be an issue.

I did some test with a build_skb() implemenation and I just discovered
that It wouldn't work... Problem of build_skb() is that the driver will
release the data and that's exactly what I want to skip (one allocated
memory space that is reused for every skb)

Wonder if it would be acceptable to allocate a skb when master became
operational and use always that.
When this preallocated skb has to be used, the required data is changed
and the users of the skb is increased so that it's not free. In theory
all the skb shared data and head should be the same as what changes of
the packet is just the data and nothing else.
It looks like an hack but that is the only way I found to skip the
skb_free when the packet is processed. (increasing the skb users)

Since the MDIO layer does not really allow more than one outstanding
transaction per MDIO device at a time, you might be just fine with just have
a front and back skb set of buffers and alternating between these two.

Another way as you suggested would be have 2 buffer and use build_skb to
use build the sbk around the allocated buffer. But still my main concern
is if the use of manually increasing the skb user is accepted to skip
any skb free from happening.

Hope I'm not too annoying with these kind of question.

-- 
Florian

-- 
	Ansuel